The choices for back to bare metal restorations

[jonathon]

This information is produced by Adrian who runs SPI in Dudley. He has kindly given permission for me to present this information in order that 'one' can make an informed decision about bare metaling your pride and joy.


Rust Removal

Whilst the process we advertise will deal effectively with the vast majority of the rust on the shell and panels including that found in the seams, recesses and box sections of a shell there may be small areas that may not have been in contact with the cleaning solutions or where they were previously very rusty then there may remnants of this rust which must be physically removed and this may involve opening up the area to expose the remaining rust for effective removal. As such we do recommend that the shell and panels are stripped and cleaned to bare metal before carrying out your major panel repairs, the shell is then returned for a secondary process and then electrophoretically coated. As you know this is a little more expensive but it does mean that any difficult to reach areas are highlighted, and modifications to improve penetration can be made prior to its return. It also ensures that any existing damage hidden beneath filler and paint can be identified.


Electrophoretic Coating

Once the shell and panels have undergone the cleaning stage they are then passed directly onto the Electrophoretic paint plant and painted the same day. Electrophoretic painting is recognised as the industry standard primer coating used on automotive shells panel and parts. Developed to minimise rust creep from stone chips it provides excellent anti-corrosion protection and because it is bonded to the steel surface it is very stable. It can successfully undergo a 180ºC bend test and can offer in excess of 1000 hours salt spray resistance. The shell or panel is passed through a comprehensive pre-treatment process where it is initially de-greased and then phosphated, the process is supplied by Chemetall plc – one of the largest and well respected chemical process providers in Europe. Once through the pre-treatment the shell is passed into the electrophoretic paint tank. Unlike paint it is a polymer resin held in a colloidal suspension (supplied by PPG – one of the worlds biggest and most respected paint manufacturers) and is deposited on the surface of the substrate via a process similar to electrolysis the solution is circulated at a high rate through a series of pumps and whilst the shell or part is in the tank it is tilted forwards and backwards to minimise the effect of any airlocks that may be present. The shell is then rinsed to remove excess resin before being cured at 180ºC for around 40 minutes to cross link the polymer and eradicate evolved gasses. The coating will only have adhered to clean surfaces and will not adhere to existing rust or other coatings – hence the shell needs to be effectively clean prior to the process and blasted shells would not be considered clean enough – more about blasting later. The resulting coating is very uniform and very thin (around 25 microns) and non porous. The fact that it is non-porous is very important as it is impervious to moisture penetration again unlike shop primers that can be microporous and allow moisture through the surface. This is why we recommend that the electrophoretic coating is not abraded away when preparing the shell for subsequent coats. Restoration specialists generally ensure that a primer coat or two is applied before the shell is rubbed back minimising the removal of the e-coat – again this does not appear to be the case with your shell. The photographs on your thread clearly demonstrate that large areas of e-coat have been removed prior to priming. Immediately following e-coat modern shells are seam sealed and primed before being finish painted and cavity waxes applied all within twenty four hours. Unfortunately this is not a possibility with a classic shell particularly when further panel work is required - however we do recommend that it is treated to protect the seams in the interim and again for the longer term.



Unfortunately try as we might and as with any process nothing is 100%. The claim we make is that it will remove rust and contamination from areas that no other process will reach. We make no secret of this and welcome open debate about the process in a bid for us to improve it wherever possible. We claim that the process is “the best available” and we continue to defend that position. I sincerely hope that my explanation of the processes involved can help better explain this claim. However in order to substantiate the theory behind the benefits of process it is important to compare the limitations of our process with the rather more limited effectiveness of other processes.



Media blasting

Many items are media blasted to remove old paint and if you are lucky some of the rust prior to the application of a primer. The removal of paint by abrasive methods including blasting is a somewhat hit and miss affair and is unable to effectively remove underseal. When paint and primer is applied to a surface it displays a variation in adhesion and thickness and as such some areas will require exposure to the blasting media for longer than others – this gives rise to preferential degradation of the underlying surface and sometimes the appearance of holes. Not only are you left with less metal than before you also run the risk of inducing a process known as shot peening which relies on the action of a media upon a surface to induce a stress layer resulting in plastic deformation of the substrate – in short the panel becomes stress relieved and often results in rippling – a phenomenon widely reported by classic car restorers – once this occurs the panel is usually scrap. So in his attempt to remove the paint and rust from the shell the poor old shotblaster is running the gauntlet of clean enough to paint or whoops I’ve scrapped the panel as a result. In their bid to avoid stress relieving the exposed surface is consequently left short of the required cleanliness. Most refinishing primers will invariably require application within 4 hours of achieving a Swedish cleanliness standard of SAE 2 ½ or better. To achieve this standard with medias such as sand, plastic bead or glass is very difficult without invoking the shot peening factors even without the initial time requirement of paint removal. In addition where rust is pitted on the surface ie Rust grade 3 or above then this will invariably still be present after blasting as the shadow of the blast will shield the rust from the media. The best way to explain this is to imagine standing behind a board facing the wind in a blizzard – unless the wind moves through 360 degrees chances are you won’t get any snow on you. This remaining rust will then be susceptible to moisture penetration through the subsequent coating and this often gives rise to small spots of rust coming through on panel faces – another phenomenon widely reported by classic car restorers. Medias such as Soda , wheat starch, and walnut shell reduce the risk of shot peening but are ineffective for the removal of rust. So to achieve the required level of cleanliness by blasting you will need to blast the surface with something more substantial and for a shorter period of time. On steel structures items are generally blasted with a chilled iron grit at a pressure of around 100psi – this will no doubt remove the paint and rust in a short space of time but will also render a delicate item like a car panel as scrap. Added to that blasting does not address the issues of rust in areas not accessible with a blasting nozzle and the sills and other box sections are left untreated, unpainted, and with a large amount of media debris wedged in the folds and seems of the car. Many of the softer medias will absorb moisture trapping it against the steel surface where they will accelerate the rusting process. Other medias may remain lodged in the shell preventing the ingress and worse the egress of moisture until it is finish painted and as such this often tends to pop out with the paint attached as soon as the car is driven down the road leaving exposed spots of rust on the car and without the protection of a sacrificial conversion coating (a tri-cationic zinc phosphate in the case of e-coat) the rust can spread across the panel face. We regularly receive shells and panels that have previously been restored following blast cleaning and they frequently show signs of extensive rust and decay as a result of improper surface preparation as well as good volume of media and blasting debris – which even we find hard to remove. I will not go on to discuss problems associated with contaminated media or responsible waste disposal…



Hand Stripping / Thermal Cleaning



The only other alternatives to Chemical Cleaning or Media Blasting is stripping by hand or thermal removal – I have grouped these together because they bring with them the same major drawback – Dealing effectively with Rust. Many of us have spent many hours with a tin of Nitromors or other such material attempting to remove paint. As with thermal methods it can effectively remove paint but leaves the now exposed steel surface in a rapidly oxidising state. Apart from being significantly bad for your health (its chief ingredient is Methylene Chloride and we would not be allowed to use in an industrial setting) it rapidly evaporates leaving no protection on the steel surface. Heat guns may remove the paint from the surface but rapidly oxidizes the panel face and the reverse of the panel face. Mechanical abrasion like shotblasting is also hit and miss but overwhelmingly one of the key problems is that none of these processes deal with the inaccessible areas of a complex fabrication and are unlikely to achieve the required level of surface cleanliness required by the primer manufacturers.



Rust Prevention



We always recommend that the seams and box sections of any shell or component is treated with a good quality rust preventative oil protection system as soon as possible to prevent the progress of rust and this should be repeated at regular intervals. Shells and components should be painted as soon as possible and seams should be sealed and protected from the ingress of moisture. After all rust requires three key ingredients - iron, oxygen and H2O.

[MarkyB]

Phew!
Very interesting read but it could do with breaking down into paragraphs more.